In general, as a hydraulic system for a construction machine, an open center type hydraulic system and a closed center type hydraulic system are used. There is a negative control system as the open center type hydraulic system, and there are a positive control system and a load sensing system as the closed center type hydraulic system.
Typically, in a case in which the closed center type hydraulic system for an excavator, which uses a pressure control type electrohydraulic pump, is suddenly stopped or a hydraulic cylinder reaches an end stroke during operation, a swash plate angle of the hydraulic pump is decreased.
As the swash plate angle of the hydraulic pump is decreased, a pressure peak instantaneously occurs due to the amount of oil discharged from the hydraulic pump.
The pressure peak, which occurs as described above, has an adverse effect on hydraulic components of the hydraulic system, and as a result, there are problems in that durability of the components deteriorate, and noise occurs due to friction between the hydraulic components.
To solve the aforementioned problems, a relief valve, which has constant operating pressure, is widely used. However, there is a problem in that there is no method of removing a pressure peak in a case in which the pressure peak occurs below pressure set for the relief valve.